Season the lamb pieces with salt and pepper. Place them in a saucepan and add garlic, lemon juice, butter, potatoes and one glass of water. Bring to boil and then let the meat simmer for 45 minutes. Remove lamb from saucepan and place it in a baking pan. Strain the meat's sauce and pour it in the baking pan. Let the meat bake in 190 C for 2 hour. 10 minutes before the end of time start preparing the yoghurt sauce. Mix in a bowl all the necessary ingredients. Spread the mixture on top of the baking pan and bake for 30 more minutes. Serve hot.